The present invention relates to a glass composition having a high modulus of elasticity, more particularly a glass composition which is suitable for use as an information recording medium substrate required to have high surface smoothness and a high modulus of elasticity. The invention further relates to an information recording medium substrate, an information recording medium, and an information recording device each comprising the glass composition.
A larger recording capacity and a shorter access time are required to information recording devices such as hard disks. One possible way to realize the latter requirement is to rev up the spinning speed of an information recording medium (hereinafter referred to as xe2x80x9crecording mediumxe2x80x9d for simplicity)
However, since substrates for information recording media (hereinafter referred to simply as xe2x80x9csubstratesxe2x80x9d) have a slight distortion by their own weight, the distortion makes substrates resonate considerably when a higher spinning speed is employed. Such a vibrational resonance causes a fatal crash of recording media by a collision between the medium and a magnetic head. This is the reason why the narrow gap between a magnetic head and a recording medium cannot be employed, and this prevents information recording devices from decreasing their recording capacity.
In order to reduce the bending and resonance of substrates, we should use substrates whose modulus of elasticity (Young""s modulus) is higher than that of conventional substrates. The commercial substrates made of an aluminum alloy, whose modulus of elasticity is 71 GPa, cannot cope with high rotational speeds of 10,000 rpm and above. Although there is a need to use thinner substrate for device miniaturization, aluminum substrates need to have a larger thickness so as to compensate for the deficiency in modulus of elasticity. Glass substrates made of chemically tempered soda-lime glass are also commercially available. However, Young""s modulus of this substrate is only 72 GPa and there is the same drawback as the aluminum substrates.
JP-A-10-81542 (the term xe2x80x9cJP-Axe2x80x9d as used herein means an xe2x80x9cunexamined published Japanese patent applicationxe2x80x9d) discloses a glass which is based on SiO2xe2x80x94Al2O3xe2x80x94RO (wherein R is a bivalent metal) and contains at least 20 mol % Al2O3 or MgO and from 0.5 to 17 mol % Y2O3, as a glass free from the drawback described above. JP-A-10-81540 discloses a glass having a glass transition point of 750xc2x0 C. or higher and a glass containing a rare-earth oxide such as Y2O3, Nd2O3, Gd2O3, Ho2O3, or La2O3 in an amount or 18 mol % or larger, while JP-A-10-79122 discloses a glass based on SiO2xe2x80x94Al2O3xe2x80x94RO (wherein R is a bivalent metal) and containing TiO2, ZrO2, Y2O3, or the like. International Publication WO 98/55993 discloses various glasses having a modulus of elasticity as represented by Young""s modulus of 100 GPa or higher and a density of 3.5 gxc2x7cmxe2x88x923 or lower. Furthermore, International Publication WO 98/09922 discloses a glass in which the specific modulus as determined by dividing the modulus of elasticity represented by Young""s modulus by the density is 36 GPaxc2x7gxe2x88x921xc2x7cm3 or higher.
However, since most of the glasses shown in the Examples given in the references cited above contain no alkali ingredients, they cannot be tempered by ion exchange and, hence, they do not have a sufficient breaking strength. Although glasses containing an alkali ingredient are described therein, most of these have a Young""s modulus as low as below 110 GPa and the remaining glasses, having a Young""s modulus of 110 GPa or higher, have a density as high as 3.1 gxc2x7cmxe2x88x923 or above. These glasses have a specific modulus lower than 39.0 GPaxc2x7gxe2x88x921xc2x7cm3. When such glasses are used as substrates, they bend considerably at a high rotational speed, or impose an excessive load on the drive. Specific modulus is the value obtained by dividing modulus of elasticity represented by Young""s modulus by density. Another drawback of those glasses is that since they contain any of Al2O3, MgO, CaO, TiO2, and ZrO2 in an excessively large amount, they easily devitrificates during glass production and, hence, it is hard to obtain a homogeneous glass.
It is inevitable that information recording devices will come to be more strongly required to attain a larger recording capacity, a shorter access time, etc. However, any of the conventional substrates cannot sufficiently meet these requirements.
The invention has been achieved in view of the above-described problems of conventional techniques.
One object of the invention is to provide a glass composition which has a high modulus of elasticity and a low density, i.e., has a high specific modulus.
Another object of the invention is to provide at low cost a glass composition which is suitable for use as an information recording medium substrate, for example, because it can be formed easily, is less apt to devitrificate, is suitable for mass production, and can be easily made to have high surface smoothness by polishing.
Still another object of the invention is to use this, glass composition to provide a substrate and a recording medium each contributing to an increase in recording density and a reduction in access time in information recording devices to thereby contribute to further progress in an information-oriented society.
These objects are accomplished with a glass composition comprising, in terms of mol %, 35 to 45% silicon dioxide (SiO2), 15 to 20% aluminum oxide (Al2O3), 3 to 10% lithium oxide (Li2O), 0.1 to 5% sodium oxide (Na2O), 15 to 30% magnesium oxide (MgO), 0 to 10% calcium oxide (CaO), 0 to 4% strontium oxide (SrO), 25 to 35% RO (MgO+CaO+SrO), 2 to 10% titanium dioxide (TiO2), 0.5 to 4% zirconium oxide (ZrO2), 4 to 12% TiO2+ZrO2, and 0 to 4% yttrium oxide (Y2O3).
The invention further provides a glass composition comprising, in terms of mol %, 35 to 42% silicon dioxide (SiO2), 15 to 20% aluminum oxide (Al2O3), 5 to 10% lithium oxide (Li2O), 0.1 to 2% sodium oxide (Na2O), 20 to 30% magnesium oxide (MgO), 0 to 10% calcium oxide (CaO), 0 to 4% strontium oxide (SrO), 25 to 35% RO (MgO+CaO+SrO), 5 to 10% titanium dioxide (TiO2), 1 to 4% zirconium oxide (ZrO2), 6 to 12% TiO2 +ZrO2, and 0 to 4% yttrium oxide (Y2O3).
The invention furthermore provides a glass composition comprising, in terms of mol %, 35 to 40% silicon dioxide (SiO2), 15 to 20% aluminum oxide (Al2O3), 3 to 10% lithium oxide (Li2O), 0.1 to 5% sodium oxide (Na2O), 15 to 30% magnesium oxide (MgO), 0 to 10% calcium oxide (CaO), 0 to 4% strontium oxide (SrO), 25 to 35% RO (MgO+CaO+SrO), 2 to 10% titanium dioxide (TiO2), 1 to 4% zirconium oxide (ZrO2), 4 to 12% TiO2+ZrO2, and 0 to 4% yttrium oxide (Y2O3).
In a preferred embodiment of the glass compositions of the invention described above, the ratio of the content of Li2O to that of (Li2O+Na2O) is from 0.76 to 0.99.
In another preferred embodiment of the glass compositions of the invention described above, the ratio of the content of MgO to that of (MgO+CaO+SrO) is from 0.76 to 1.00.
In still another preferred embodiment of the glass compositions of the invention described above, the ratio of the content of Al2O3 to that of (Li2O+Na2O) is from 1.5 to 4.0.
The invention still further provides a glass composition which has a modulus of elasticity as represented by Young""s modulus of 110 GPa or higher and a density lower than 3.1 g/cm3 and in which the specific modulus G as determined by dividing the modulus of elasticity by the density thereof satisfies the following relationship (1).                     G        ≥                  9.36          ⁢                                    {                                                ∑                  i                                ⁢                                                      u                    i                                    ⁢                                                            x                      i                                        /                                          d                      i                                                                                  }                        ·                                          {                                                      ∑                    i                                    ⁢                                                            v                      i                                        ⁢                                          x                      i                                                                      }                            /                              {                                                      ∑                    i                                    ⁢                                                            w                      i                                        ⁢                                          x                      i                                                                      }                                                                        (        1        )            
A further preferred embodiment of the glass compositions of the invention described above is one which has a modulus of elasticity as represented by Young""s modulus of 110 GPa or higher and a density lower than 3.1 g/cm3 and in which the specific modulus G as determined by dividing the modulus of elasticity by the density thereof is 39.0 GPaxc2x7gxe2x88x921xe2x80x83cm3 or higher.
Still a further preferred embodiment of the glass compositions of the invention described above is one which has undergone an ion-exchange treatment in a molten salt containing at least either of potassium ions and sodium ions.
The invention still further provides a substrate for information recording media which comprises any of the glass compositions described above.
The invention still further provides an information recording medium employing the substrate for information recording media described above.
The invention still further provides an information recording device employing the information recording medium described above.
Modes for carrying out the invention will be explained in below in detail. Hereinafter, all percents are by mol, and xe2x80x9cmodulus of elasticityxe2x80x9d means Young""s modulus and xe2x80x9cspecific modulusxe2x80x9d means the value obtained by dividing the modulus of elasticity representing Young""s modulus by density.
The present inventors made intensive investigations on the relationship among composition, modulus of elasticity, and specific gravity in oxide glasses. As a result, they have succeeded in developing a glass composition having a modulus of elasticity of 110 GPa or higher, a density lower than 3.1 gxc2x7cmxe2x88x923, and a specific modulus of 39.0 GPaxc2x7gxe2x88x921xc2x7cm3 or higher by adding suitable amounts of TiO2 and ZrO2 to a glass composition comprising SiO2, Al2O3, Li2O, Na2O, and MgO as essential components. Each component of the glass composition is contained in an amount within the range shown below and performs its functions.
SiO2 is an essential component constituting the network structure of the glass. The silica glass consisting of SiO2 alone has a modulus of elasticity of about 70 GPa; therefore as the content of SiO2 increases, the modulus of elasticity approaches that value. Consequently, in order to improve modulus of elasticity of the glass, there is an upper limit in SiO2 content. Contents of SiO2 not higher than 45% result in a modulus of elasticity of 110 GPa or higher. When a substrate made of a glass composition having a modulus of elasticity of 110 GPa or higher is used, an information recording device is obtained which stably works at a magnetic head/recording medium gap (flying height) of 15 nm and a rotational speed of 10,000 rpm or higher. In the case where the SiO2 content is 40% or lower, a modulus of elasticity of 110 GPa or higher is obtained more surely.
Even in the case where the SiO2 content is higher than 40% but not higher than 42%, a modulus of elasticity almost the same as in the case where the SiO2 content is 40% or lower can be attained by regulating the contents of the other components. From the standpoint of glass formability, the upper limit of SiO2 content is preferably high because the viscosity of a glass melt fluctuates depending on the SiO2 content thereof. In the case where the SiO2 content is 42% or lower, it is preferred to heighten the contents of Li2O, MgO, and TiO2, which each increases the modulus of elasticity, and to lower the content of Na2O, which less contributes to an improvement in modulus of elasticity.
On the other hand, too low SiO2 contents make glass formation impossible. Consequently, the lower limit of SiO2 content should be within a range where glass formation is possible. By regulating the contents of components other than SiO2 to respective values within the ranges which will be given later, an SiO2 content of 35% or higher can be attained. Consequently, the content of SiO2 in the glass composition of the invention should be from 35 to 45%, and is preferably from 35 to 42%, more preferably from 35 to 40%.
Al2O3 is an essential component which improves the modulus of elasticity and water resistance of the glass composition and increases the thickness of a compression stress layer formed by ion exchange. In case where the content of Al2O3 is lower than 15%, these effects are insufficient. On the other hand, contents thereof exceeding 20% result in impaired meltability. In this case, however long a batch is heated, complete melting may be impossible. Even when the batch has melted completely, this composition is highly susceptible to devitrification upon cooling/solidification and glass formation is difficult. Consequently, the content of Al2O3 in the glass composition should be from 15 to 20%. The term xe2x80x9cbatchxe2x80x9d means a mixture prepared by mixing raw glass materials together in such a proportion as to result in a glass made up of components contained in respective given amounts.
In case where the proportion of Al2O3 to all alkali ingredients (Li2O+Na2O) is smaller than 1.5, the glass is apt to be not transparent but milk-white. On the other hand, in case where that proportion exceeds 4.0, the glass is highly apt to devitrificate. Consequently, Al2O3/(Li2O+Na2O) is preferably from 1.5 to 4.0.
Li2O is an essential component which effectively improves the modulus of elasticity of the glass composition and undergoes ion replacement in ion exchange conducted for strengthening. It serves also to lower the melting temperature of the glass and thus enhance meltability. Li2O is thought to further serve to increase the rate of polishing for obtaining a mirror surface. Because of these, the content thereof is preferably as high as possible. The lower limit of Li2O content is 3%. Contents of Li2O lower than 3% only result in a composition having a Young""s modulus lower than 110 GPa. Such too low Li2O contents also result in insufficient strength of the glass composition because the amount of ions which are replaced in an ion-exchange treatment is insufficient.
On the other hand, when the content of SiO2 is 40% or higher, the content of Li2O is preferably 5% or higher from the standpoint of avoiding a decrease in modulus of elasticity. Although Li2O is an ingredient having excellent properties as described above, it strongly tends to make the glass milk-white in glass compositions having a low SiO2 content and a high Al2O3 content. Because of this, the upper limit of Li2O content is preferably 10%. Consequently, the content of Li2O in the glass composition should be from 3 to 10%, and is preferably from 5 to 10%.
Na2O is an essential component which, like Li2O, undergoes ion replacement in an ion-exchange treatment. Although less effective than Li2O in improving modulus of elasticity, Na2O is superior in lowering the melting temperature and liquidus temperature of the glass to enhance meltability and in heightening the rate of mirror polishing. Especially when incorporated in a small amount (e.g., 0.1%), Na2O greatly heightens the stability of the glass. On the other hand, incorporation of Na2O in too large an amount (e.g., 10%) results in reduced contents of the other ingredients serving to improve modulus of elasticity. Consequently, the content of Na2O in the glass composition should be 5% or lower. In the case of a glass composition having a high SiO2 content (40% or higher), the content of Na2O is preferably 2% or lower because of the necessity of heightening the contents of the other ingredients serving to improve modulus of elasticity as stated above. Consequently, the preferred range of Na2O content is from 0.1 to 5%, and the more preferred range thereof is from 0.1 to 2%.
In case where the total content of Li2O and Na2O (R2O) in a glass is lower than 5%, this glass undergoes insufficient ion exchange and has poor meltability. On the other hand, in case where the total content thereof exceeds 12%, the glass is apt to be milk-white and suffer devitrification and is less apt to transparent. Consequently, the content of R2O in the glass composition is preferably from 5 to 12%.
Furthermore, in case where the proportion of Li2O to R2O (Li2O/(Li2O+Na2O)) is smaller than 0.76, the glass has a reduced modulus of elasticity. However, in case where that proportion is 1, namely when the glass contains no Na2O, the glass has impaired stability and the glass melt is apt to suffer devitrification upon cooling/solidification. Consequently, Li2O/(Li2O+Na2O) is preferably 0.76 or larger and should be 0.99 or smaller.
MgO is the most important essential component which serves as a network-modifying oxide in the glass. MgO is an ingredient which improves the modulus of elasticity of the glass, reduces the density thereof, accelerates batch melting, and enhances meltability. The higher the content of MgO, the more these functions are exhibited. However, MgO contents higher than a given value result in a glass melt which has an excessively reduced viscosity and undergoes devitrification upon cooling/solidification, making glass formation impossible. Because of this, the content of MgO should be 30% or lower. On the other hand, the lower limit of MgO content is determined while comparing the effect of improving modulus of elasticity with the effect of reducing glass density. Specifically, the lower limit thereof should be 15%. However, in the case of a glass composition having a high SiO2 content (40% or higher), the content of MgO is desirably 20% or higher from the standpoint of maintaining a modulus of elasticity. Consequently, the content of MgO in the glass composition should be from 15 to 30%, and is preferably from 20 to 30%.
CaO is an optional component which, like MgO, improves modulus of elasticity and enhances batch meltability. Although superior to MgO in enhancing the devitrification resistance of a glass, CaO has a drawback that it slightly increases the density of the glass composition. Because of this, an increase in density and deterioration in devitrification resistance are taken in account in determining the content of CaO. Specifically, the content of CaO in the glass composition should be 10% or lower, and is preferably 6% or lower. On the other hand, the content of CaO is preferably 0.1% or higher because those effects are produced when CaO is incorporated even in a small amount.
SrO is an optional component which, like MgO and CaO, enhances batch meltability. When incorporated even in a small amount (e.g., 0.1%), SrO greatly improves the devitrification resistance of the glass. However, the content of SrO is regulated to a small value because SrO not only is less effective than MgO and CaO in improving the modulus of elasticity of the glass composition but considerably increases the density of the glass. In order to regulate the glass composition so as to have a density lower than 3.1 gxc2x7cmxe2x88x923, the content of SrO should be regulated to 4% or lower. On the other hand, SrO is exceedingly superior to other components in the function of improving devitrification resistance. Since recovery of devitrification resistance is important especially in the presence of CaO, it is preferred that SrO be contained in an amount of at least 0.1%.
In case where the total content of all the bivalent-metal oxide components RO (RO=MgO+CaO+SrO) is lower than 25%, the glass composition frequently has a modulus of elasticity lower than 110 GPa and batch meltability is insufficient. As the content of RO increases, the modulus of elasticity increases. However, as the content thereof increases beyond 35%, the devitrification resistance of the glass abruptly becomes worse. In this case, the glass melt frequently suffers devitrification upon cooling/solidification, making it impossible to give a homogeneous glass. Consequently, the content of RO in the glass composition should be from 25 to 35%.
Furthermore, in case where the proportion of MgO to RO, i.e., MgO/(MgO+CaO+SrO), is smaller than 0.76, the glass composition has a reduced modulus of elasticity and an increased density. Consequently, the lower limit of MgO/(MgO+CaO+SrO) is preferably 0.76. On the other hand, MgO/(MgO+CaO+SrO) may be 1.00 because CaO and SrO are optional components. However, the upper limit thereof is preferably 0.99 in view of the excellent functions of CaO and SrO.
TiO2 is an essential component which heightens the modulus of elasticity of the glass composition and improves the weatherability thereof. As apparent from the fact that a relatively large amount of TiO2 is contained in milk-white glasses, TiO2 has the function (side-effect) of making a glass milk-white. Because of this, the content thereof should be 10% or lower. On the other hand, the content thereof should be at least 2% from the standpoint of enabling the TiO2 to fully perform its function of improving modulus of elasticity. In the case of a glass composition having a high SiO2 content (40% or higher), the TiO2 content is preferably 5% or higher from the standpoint of preventing a decrease in modulus of elasticity. Consequently, the content of TiO2 in the glass composition should be from 2 to 10%, and is preferably from 5 to 10%.
ZrO2 is an essential component which, like TiO2, heightens the modulus of elasticity of the glass composition and improves the weatherability thereof. As apparent from the fact that ZrO2 is used as a nucleating agent for crystallized glasses, ZrO2 accelerates glass crystallization. ZrO2 further has the function (side-effect) of increasing the density of the glass composition. Consequently, the content of ZrO2 should be 4% or lower from the standpoint of avoiding unnecessary crystallization (devitrification) and an increase in density. On the other hand, from the standpoint of enabling those desirable functions to be fully performed, the content thereof should be 0.5% or higher and is preferably 1% or higher. Consequently, the content of ZrO2 in the glass composition should be from 0.5 to 4%, and is preferably from 1 to 4%.
TiO2 and ZrO2, when coexistent with each other, produce a synergistic effect to greatly improve the modulus of elasticity of the glass composition. However, because of the undesirable functions (side-effects) of TiO2 and ZrO2, the glass composition is apt to suffer devitrification and to become milk-white when the total content of TiO2 and ZrO2 exceeds 12%. On the other hand, from the standpoint of enabling the synergistic effect of TiO2 and ZrO2 to be produced, the total content of TiO2 and ZrO2 should be 4% or higher and is preferably 6% or higher. Consequently, the total content of TiO2 and ZrO2 in the glass composition should be from 4 to 12%, and is desirably from 6 to 12%.
Y2O3 is an optional component which improves the modulus of elasticity of the glass composition and simultaneously increases the density thereof greatly. Since incorporation of Y2O3 necessitates use of an expensive raw glass material, the content of Y2O3 is preferably low from the standpoint of cost. In addition, as the content thereof increases beyond 4%, the function of improving modulus of elasticity decreases abruptly. Consequently, the content of Y2O3 in the glass composition should be 4% or lower.
Besides the components described above, other ingredients may be contained in a total amount of up to 3% for the purposes of coloring, melt clarification, etc. Examples of such optional ingredients include As2O3, Sb2O3, SO3, SnO2, Fe2O3, CoO, Cl, and F. There are cases where ingredients including K2O which are contained as impurities in minute amounts in industrially utilizable raw glass materials come into the glass composition. As long as the total content of these impurities is lower than 3%, they exert little influence on the properties of the glass composition and pose practically no problems.
When a glass composition comprises the components described above whose contents are within the respective ranges shown above, it has a modulus of elasticity of 110 GPa or higher, a density lower than 3.1 g/cm3, and a specific modulus G satisfying the following relationship (1).                     G        ≥                  9.36          ⁢                                    {                                                ∑                  i                                ⁢                                                      U                    i                                    ⁢                                                            X                      i                                        /                                          d                      i                                                                                  }                        ·                                          {                                                      ∑                    i                                    ⁢                                                            V                      i                                        ⁢                                          X                      i                                                                      }                            /                              {                                                      ∑                    i                                    ⁢                                                            W                      i                                        ⁢                                          X                      i                                                                      }                                                                        (        1        )            
In relationship (1), Ui is the dissociation energy (energy required for decomposition into ions) for component i, and the unit thereof is kcal/mol; Xi is the molar fraction of component i; di is the density of component i, and the unit thereof is g/cm3; and Vi is that volume in the glass which is occupied by component i, a compound represented by the general formula AxOy, and which is represented by the following equation (2).
Vi=NA4/3xcfx80(xRA3+yRo3)xe2x80x83xe2x80x83(2)
In equation (2), NA is the constant called Avogadro""s number (6.022xc3x971023); xcfx80 is the ratio of the circumference of a circle to its diameter, i.e., 3.14; and RA and RO are the ionic radii of metal A and oxygen O, respectively. Values of RA and Ro are reported, e.g., in L. Pauling, Nature of Chemical Bond and Structure of Molecules and Crystals, 3rd. ed., Cornell University Press, Ithaca, N.Y. (1960); R. D. Shannon, Acta Crysta, Vol.A32, p.751 (1976); and C. T. Prewitt, Acta Crysta, Vol.B25, p.925 (1969), and soon. The unit of RA and Ro is angstrom. Furthermore, Wi is the molecular weight of component i.
In relationship (1), the right side is calculated from known data for the components. As apparent from each of the Examples which will be given later, the calculated value well approximates to the specific modulus of the glass composition. Consequently, the specific modulus of a glass composition can be predicated by calculating the right side of relationship (1).
The glass composition satisfying relationship (1) has an exceedingly high specific modulus and can sufficiently have the performances required of next-generation substrates. Namely, the substrates made of this glass composition can attain a flying height of 10 nm. On the other hand, glass compositions not satisfying relationship (1) are ones which are apt to undergo devitrification or become milk-white and have poor formability or ones which have a low specific modulus, as will be described in Examples. Consequently, relationship (1) can be a good index to a judgement as to whether a glass composition is suitable for use as a substrate.
When the contents of components in a glass composition are further regulated to values within respective ranges specified above, the glass composition formed can have a modulus of elasticity of 110 GPa or higher, a density lower than 3.1 g/cm3, and a specific modulus of 39.0 GPaxc2x7gxe2x88x921xc2x7cm3 or higher. Furthermore, it has been ascertained, from Examples which will be given later, that modulus of elasticity, density, and specific modulus are improved to 124 GPa, 2.75 g/cm3, and 41.2 GPaxc2x7gxe2x88x921xc2x7cm3, respectively.
Since this glass composition contains Li2O and Na2O, it can be chemically strengthened through an ion-exchange treatment. This ion-exchange treatment is accomplished by immersing the glass composition at a temperature not lower than the strain point thereof in a molten salt containing at least either of potassium ions and sodium ions. As a result of this chemical strengthening, a compression stress generates on the surface of the glass composition to improve the fracture strength thereof.
Furthermore, since this glass composition is a homogeneous transparent glass containing alkali components (R2O) in a moderate amount, it can be mirror-polished with a general polishing technique for soda-lime glasses at a rate at least two times the rate of polishing for conventional high-modulus glasses. The polished mirror surface of this glass composition has high surface smoothness comparable to that of conventional high-modulus glasses.
Therefore, the substrate comprising this glass composition has a higher modulus of elasticity and a lower density than conventional substrates. Namely, the substrate has extraordinarily high specific modulus. Because of this, the substrate is less weighed down by itself and is less apt to suffer vibration caused by resonance. Furthermore, the substrate can be easily toughened by chemical strengthening and can be made to have high smoothness through polishing. This substrate can hence be easily mass-produced with conventional production apparatus. The recording medium containing this substrate also has high performances and can be mass-produced at low cost. The information recording device containing this recording medium has a high recording density and a reduced access time.